1. Field of the Invention
The present invention relates to a display device and a driving method for a display panel, and more particularly a method to adjust a gray-scale level displayed on the display panel as desired by performing a correction to a gray-scale data.
2. Description of the Related Art
In a liquid crystal display, a gamma correction is performed in accordance with voltage-transmission characteristics (V-T characteristics) of a liquid crystal panel to correct a corresponding relationship between a gray-scale data supplied from an outside and a driving signal for driving a display device. Since the V-T characteristics are nonlinear, a nonlinear driving voltage needs to be generated by a gamma correction with respect to a value of gray-scale data in order to display an original image in a correct color tone. Moreover, a gamma correction is performed by occasionally using different gamma values for R (red), G (green) and B (blue) respectively in order to improve the color tone of a display image. Since each of R (red), G (green) and B (blue) has different voltage-transmission characteristics of the liquid crystal panel, it is preferable to perform the gamma correction by using a gamma value corresponding to the color for the improvement of the color tone of the display image.
There are roughly two methods to realize the gamma correction in the liquid crystal panel. One method (hereinafter referred to as the first method) controls a gray-scale voltage corresponding to each of usable gray-scales to a voltage level corresponding to a gamma curve. The driving voltage of the liquid crystal panel is generated by generally selecting a gray-scale voltage corresponding to a gray-scale data from a plurality of gray-scale voltages. Accordingly, a gamma correction is realized by controlling the voltage level of each gray-scale voltage to meet with the gamma curve.
The other method (hereinafter referred to as the second method) executes a data processing for gray-scale data. In the gamma correction, the data processing is executed in accordance with the following formula with respect to input gray-scale data DIN so as to generate corrected gray-scale data Dγ.Dγ=DγMAX(DIN/DINMAX)γ,  (1)A driving voltage for driving a signal line is generated in accordance with the corrected gray-scale data Dγ that was generated beforehand.
There are positive and negative aspects in the first and second methods. In the first method, since a gray-scale voltage applied to the liquid crystal panel is adjusted in consideration with the V-T characteristics of the liquid crystal panel, a precise correction can be realized for various gamma curves. However, it is difficult for the first method to adjust a gray-scale voltage, and it is not suitable to perform a gamma correction with different gamma values in R (red), G (green) and B (blue) respectively. It is because the gray-scale voltage provided in the inside of a driver IC which drives a signal line of the liquid crystal panel is shared among R (red), G (green) and B (blue); and if it is assumed to change the gray-scale voltages respectively for R (red), G (green) and B (blue), signal lines for supplying a gray-scale voltage need to be provided separately in each of R (red), G (green) and B (blue). Meanwhile, it is suitable for the second method to perform a gamma correction with different gamma values for R (red), G (green) and B (blue) respectively. However, in the second method, a circuit size tends to be large.
It is especially problematic in the second method that an arithmetic operation including exponentiation is involved in the formula (1). A circuit for rigorously executing the arithmetic operation of exponentiation is complicated and has a problem of being mounted to a liquid crystal driver. If a device has an excellent arithmetic operation capability such as CPU (Central Processing Unit), the arithmetic operation of exponentiation can be rigorously executed by a combination of a logarithmic operation, multiplication and exponential operation. For example, Japanese Laid-Open Patent Application JP-P2001-103504A discloses a mounting method of a gamma correction which is realized by a combination of a logarithmic operation, multiplication and exponential operation. However, it is not preferable to mount a circuit for rigorously executing exponentiation in terms of reducing a hard ware.
One of the simple mounting methods for the gamma correction is to use a look-up table (LUT) in which the corresponding relationship between the input gray-scale data and the corrected gray-scale data is written. The gamma correction can be realized without directly executing exponentiation by defining the corresponding relationship between the input gray-scale data and the corrected gray-scale data written in the LUT in accordance with the formula (1). Japanese Laid-Open Patent Application JP-P2001-238227A and JP-A-Heisei 07-056545 disclose a technique to prepare the LUTs for R (red), G (green) and B (blue) respectively in order to perform the gamma correction corresponding to gamma values which are different in the respective colors.
One of the problems to perform the gamma correction by using the LUT is that the size (or the number) of the LUT needs to be increased to perform the gamma correction corresponding to the different gamma values. For example, in order to perform the gamma correction for each of R, G and B and for 256 kinds of the gamma values by using the LUT with the 6-bit input gray-scale data and the 8-bit corrected gray-scale data, the LUT needs to have 393216 (=64×8×3×256) bits. It is problematic on mounting the gamma correction to the liquid crystal driver.
Japanese Laid-Open Patent Application JP-A-Heisei 09-288468 discloses a technique to perform the gamma correction corresponding to a plurality of the gamma values while sustaining the LUT size small. In this technique, a liquid crystal display device is provided with the rewritable LUT. Data to be stored in the LUT are calculated by a CPU using arithmetic operation data stored in an EEPROM, and then transmitted from the CPU to the LUT. Japanese Laid-Open Patent Application JP-P2004-212598A also discloses a similar technique. According to the technique described there, the LUT data is generated by a brightness distribution determination circuit and transmitted to the LUT.
Japanese Laid-Open Patent Application JP-P2000-184236A discloses a technique to suppress the increase of the circuit size by using the LUT, in which the corresponding relationship between the input gray-scale data and the corrected gray-scale data is written, for calculating polygonal line approximation parameters instead of directly using for generating the corrected gray-scale data. In this technique, the corrected gray-scale data corresponding to specific gray-scale data are calculated by using the LUT so as to calculate polygonal line graph information including the polygonal line approximation parameters by using the corrected gray-scale data calculated above. When the input gray-scale data is provided, the corrected gray-scale data are calculated by the polygonal line approximation indicated in the polygonal line graph information.
However, in the conventional technique, it is impossible to instantly switch gamma curves (i.e. an instant switch of gamma values for a gamma correction) in accordance with the changes of a surrounding environment of a liquid crystal display. Since portable terminals such as a laptop PC, PDA (Personal Data Assistant) and a mobile phone can be used under various environments, there is a demand to change the visibility of the liquid crystal panel to correspond to the environmental changes. For example, in a liquid crystal display using a semi-transmission liquid crystal, a reflection mode is used to display images when the intensity of the external light is strong, and a transmission mode is used to display images when the intensity of the external light is weak. Since the reflection mode and the transmission mode have different gamma values in the liquid crystal panel, the visual performance of the liquid crystal highly depends on the intensity of the external light. Therefore, if it is possible to instantly switch the gamma values by corresponding to the intensity of the external light, the visibility of the liquid crystal display can be significantly enhanced. However, conventional techniques are unable to satisfy these demands. For example, in a technique described in Japanese Laid-Open Patent Application JP-A-Heisei 09-288468 and Japanese Laid-Open Patent Application JP-P2004-212598A, data to be stored in the LUT needs to be transmitted to the LUT and the LUT needs to be rewritten so as to switch the gamma values for the gamma correction. Because of a considerable size of the data stored in the LUT, it is still difficult to instantly switch the LUT. It means that the gamma values are difficult to be switched instantly for the gamma correction.
Based on these situations, it is now demanded to provide a technique which can instantly switch the correction curves (e.g. gamma curves for performing the gamma correction) in a short period of time in accordance with the change of a surrounding environment in a display device, while a circuit size is kept to be small.